1. Field of the Invention
The present invention relates to a power amplifier, and more specifically, to a heterojunction bipolar transistor (HBT) power amplifier integrated circuit.
2. Description of the Prior Art
Power amplifier integrated circuits using HBTs are suitable for a wide range of applications and are particularly well adapted for use as high power microwave amplifiers such as those used in mobile phones.
A frequent and often serious problem with HBT power amplifiers is excessive heat buildup. Power amplifier integrated circuits operate at high current density, and hence high power density, and thus, heat generated by devices of the HBT elevates junction temperature significantly above ambient temperature. High junction temperature degrades the device reliability and limits the maximum power density of the device. Additionally, operating at higher power density risks thermal runaway of the power amplifier, in which the power amplifier suffers catastrophic device failure. Furthermore, operating at a higher junction temperature reduces device mean time to failure (MTTF). Typically, for a given application, larger devices are required to overcome this problem, leading to increased cost and inefficient use of space.
Adlerstein et al. in U.S. Pat. No. 5,986,324, which is incorporated herein by reference, describes in detail an HBT structure and operation thereof. Miura et al. in U.S. Pat. No. 5,793,067, which is also incorporated herein by reference, teaches how a transistor structure can be made with widened leads to reduce thermal resistance. However, both Adlerstein et al. and Miura et al. teach the use of an emitter air-bridge that is costly and causes undue fabrication complexity.
The prior art heat dissipation in power amplifier transistors, such as HBTs, is inadequate. Moreover, prior art solutions providing heat dissipation are difficult and costly to manufacture, impacting yield and reliability. Such insufficient heat dissipation prevents prior art power amplifiers from operating at high current densities or high power, and dictates a larger power amplifier for a given application. Finally, the cost associated with using the air-bridge for cooling is also much higher compared to a die without the air-bridge.